1. Field of the Invention
The invention concerns a screen for cathode ray tubes with traces of high luminance and color which can be adjusted by the acceleration voltage of the beam. In particular, it concerns the structure of this screen. It also concerns a method for making this screen.
2. Description of the Prior Art
In the prior art, there are so-called penetration cathode tube luminescent screens wherein the color of the trace varies with the acceleration voltage of the electron beam.
These luminescent cathode screens have two luminophors with different fluorescence levels, which shall be taken to be green and red in the rest of the description. They are represented schematically in the form of two homogeneous layers, each consisting of one of the luminophors, which may or may not be separated by an inert or non-luminescent material emitting no light under the effect of an electron bombardment.
Depending on the value of the beam acceleration voltage V, i.e. depending on the energy of the electrons of the beam, only the first of these layers, which will be assumed to be the layer with red fluorescence in the rest of this description, is excited by the beam, or else, if this voltage is sufficient, this layer or the entire second layer or a part of the second layer, made of a luminophor with green fluorescence, undergoes excitation by the beam. The green fluorescence begins to be excited only from a certain value of this voltage onwards, namely from the voltage sufficient for the energy of the electrons to enable them to penetrate the screen (whence the above expression `penetration screens`) up to the green luminophor layer, after crossing the red luminophor layer and, as the case may be, the inert layer. This value of the voltage shall be called V.sub.o in the rest of the description. The green fluorescence predominates and the trace takes on the color green. Between the two values, V.sub.o and V.sub.1, there are obtained, depending on the value of V, intermediate colors between green and red depending on the proportions of the two excited fluorescences in the trace.
There are various known prior art penetration screen structures.
One of them comprises layers of red and green luminophors separated by a layer of an inert material, all obtained by evaporation under vacuum in the form of thin films (Cf. patent on behalf of Feldmen, delivered in the United States of America with the number 3 225 238). This type of screen has the drawback of displaying extremely low luminance under electron bombardment because it is impossible for the radiation emitted in these films to emerge from them owing to the multiple reflections that they undergo.
Another of these structures has a mixture of crystals of the two luminophors, red and green, the crystals of this mixture having been coated, prior to the mixing operation, with a film of inert material. Generally, with these mixtures, a luminance is obtained which is substantially greater than that of the previous structure.
In a third prior art structure (see the U.S. Pat. No. 3,714,490 delivered in the U.S.A.), the red luminophor is integrated in the structure in the form of small-sized grains surrounding the inert layer coating bigger grains of green luminophor. The luminance of the red luminophor is thus substantially improved, but the problem to be resolved is really that of increasing the total luminance of the screen, namely luminance of all the traces and not only that given by the red luminophor. At the same time as this red luminophor luminance is increased, the luminance of the green luminophor should be increased in the same proportions for it is this proportion which sets the extent of the range of shades obtained between red and green from the moment when the green fluorescence begins to be excited by the acceleration voltage, namely from the value V.sub.o above. It is generally desired to have this entire range available in order to display all the information. This range is narrowed down if the increase in the luminance of the red luminophor is not accompanied by that of the green luminophor.
In a fourth prior art structure, as shown in FIG. 1, the screen consists of superimposed layers and, as set out in detail further below, it has the following elements:
a transparent support 1, made of glass, forming the screen proper; PA1 a layer 2 of a green luminophor made up of crystals 10, said layer being deposited according to a well known sedimentation technique; PA1 a barrier layer 3 comprising a transparent inert material. This layer is conventionally made either of zinc sulphide (ZnS) or silicon oxide (Si0.sub.2); PA1 a layer 4 of a red luminophor made up of crystals 40, said layer being deposited either by centrifugation, with the support 1 rotating on an axis parallel to its plane, or by fixing the screen on a spinner, the axis of which is the same as that of the screen. The thickness deposited depends on the operating voltages of the tube. The crystals are either yttrium vanadate or yttrium oxysulphide or gadolinium oxide doped with europium; PA1 and finally, a thin conductive layer, generally made of aluminium. This layer is taken to the high voltage of the tube and enables the removal of the electrostatic charges. It is also used as a reflector in order to obtain a one-directional radiation. PA1 an organic film is made by known methods on the layer of green luminophor, said film acting as a temporary support for the inert material forming the barrier layer 3. The material used is generally a polymer, butyl metacrylate, and it is formed by wet deposition on the screen of a solution containing this product. After the water evaporates, the film is bonded to the layer of green luminophor. Another approach uses a nitrocellulose film made in a standard way according to the prior art; PA1 then the inert material is deposited by vacuum evaporation, either by Joule effect or by an electron gun. The thickness deposited is measured and adjusted by known means in order to achieve the desired thickness which makes it possible to obtain the desired threshold voltage forming a barrier to electrons with excessively low energy. PA1 depositing the first layer of luminophor on the transparent screen; PA1 depositing an organic film on this first layer of luminophor; PA1 depositing the first layer of silicon oxide; PA1 depositing the layer of zinc sulphide on the layer of silicon oxide; PA1 depositing the second layer of silicon oxide on the layer of zinc sulphide, the latter three layers forming an inert material constituting a barrier against the electrons which have an acceleration voltage below the voltage threshold forming this barrier; PA1 depositing the first layer of luminophor on the second layer of silicon dioxide; PA1 depositing an electrically conductive layer; PA1 eliminating the organic layer by heat treatment of the structure.
The barrier layer 3 is deposited after the deposition of the first luminophor 2 in the following way:
The temporary organic film is removed during the subsequent heat treatments applied to the cathode tube.
The deposition of this temporary supporting film is indispensable for the inert layer to have a plane surface and a thickness which is as constant as possible throughout its range, in order to obtain proper functioning of the screens.
Now, following the manufacture of screens with this latter structure, it has been observed that the characteristics of these screens are not as good as expected. For, after the observation of the structure, it has been observed, in fact, that the barrier layer deteriorates during manufacture, and this deterioration takes the form of cracks in the inert material.
For, during the vacuum evaporation, the inert material, when it is silicon oxide, undergoes stretching stresses which cause cracks, and also cause tearing in the organic film.
When the material is zinc sulphide, the stresses undergone by this layer are compressive stresses, and the layer has no cracks. This water-sensitive material no longer has good optical transmission after having undergone all the screen finishing operations.
Furthermore, the high index of zinc sulphide (n=2.3) limits optical transmission, especially that of red radiation coming from the red luminophor.
The present invention makes it possible to overcome all these drawbacks. An object of the present invention is a high luminance color screen for cathode ray tubes, wherein the barrier layer has all the following characteristics: high quality optical transparency, uniform thickness adapted to the stopping of electrons, sound mechanical solidity and chemical stability under vacuum, under electron bombardment, and in the presence of various products such as water or organic solvents.